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<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <!-- 请应⽤ prototype 写⼀个类继承多个类的案例 -->
</head>
<body>
  <script>
    function ClassA(){
      this.a = 'a';
    }
    function ClassB(){
      this.b ='b';
    }
    ClassB.prototype = new ClassA();
    var objB = new ClassB();
    for(var p in objB){
      console.log(p);
    }
  </script>
</body>
</html>